[PATCH android/bionic 3/3] Hide libm private symbols using GCC visibility pragma

Following the visibility changes in libc.so, this patch attempts to hide private symbols in libm.so.

Reference code size measurements for HTC Passion: $ arm-eabi-size out/target/product/passion/system/lib/libm.so

[before] text data bss dec hex filename 88423 256 32 88711 15a87 out/target/product/passion/system/lib/libm.so

[after] text data bss dec hex filename 87950 256 32 88238 158ae out/target/product/passion/system/lib/libm.so

Code Review: https://review.source.android.com/#change,17358